Electrode Essentials

What battery electrodes are made of, the difference between cathode and anode, and why positive/negative terminology can be confusing.

Electrodes are where lithium is stored, and the difference in electrochemical potential between them is what enables a battery to deliver energy. This page breaks down what electrodes are and why the terminology used to describe them is not as straightforward as it seems at first glance.

Electrode Structure

Electrodes for lithium-ion batteries are porous thin layers made of materials that can store lithium through intercalation. When the battery is fully charged, lithium is predominantly stored in the negative electrode. During discharge, lithium ions move from the negative electrode to the positive electrode while electrons flow through the external circuit.

Electrodes are not only made of the particles that can store lithium (called active materials) but also include:

• Binder: Holds the electrode together
• Additives: Improve performance, for example by increasing electronic conductivity

The Two Electrodes

Positive Electrode

Also known as the cathode. This electrode has a higher open-circuit potential, and lithium ions move towards it during discharge. The active material is typically a lithium-metal oxide (e.g., lithium cobalt oxide or lithium iron phosphate), though research continues into novel materials that can improve battery performance and lifetime.

Negative Electrode

Also known as the anode. This electrode has a lower open-circuit potential, and lithium ions move away from it during discharge. The active material is usually graphite, sometimes with silicon or silicon oxide additions. Silicon offers higher intercalation capacity and potentially much higher energy densities, but its large expansion during lithiation (up to four times its initial volume) causes significant battery life issues.

Why “Positive/Negative” Instead of “Cathode/Anode”?

In Ionworks and PyBaMM, we use “positive electrode” and “negative electrode” instead of “cathode” and “anode,” which are widely used in the literature. Here’s why:

The terms “anode” and “cathode” (coined by William Whewell at the request of Michael Faraday) describe electrodes based on the direction of current flow: during discharge, the anode is where oxidation occurs and the cathode is where reduction occurs.

In common usage, “anode” refers to the negative electrode and “cathode” to the positive electrode. This is correct during discharge. However, during charge, the roles reverse: the negative electrode becomes the cathode (reduction occurs) and the positive electrode becomes the anode (oxidation occurs).

Despite the well-established convention, using anode/cathode is not correct from an electrochemical point of view. The positive/negative electrode nomenclature is more accurate:

Nomenclature	Definition
Positive electrode	Has the higher open-circuit potential
Negative electrode	Has the lower open-circuit potential

This leads us to another extremely important concept in lithium-ion batteries: the open-circuit potential.

Related Topics

• How Do Batteries Work?—overview of lithium-ion battery operation
• Open-Circuit Voltage—how electrode potentials determine battery voltage
• Reaction Kinetics—how quickly reactions occur at electrode surfaces
• Mechanical Degradation—how electrode materials degrade over time